1. Field of the Invention
This invention relates to optical imaging, and to a system for producing a displayable image based upon scanned radiation from a scene. More particularly, this invention relates to an integrated thermal imaging system wherein several modules are integrated into a single compact unit.
2. Background Art
Early thermal imagers were serial scan devices. In serial scan devices, a single detector element is scanned optically such that it sequentially receives radiation from each picture element or "pixel" in the scene. This type of serial scan device is described in Anderson, R. F., "TV Compatible Forward Looking Infrared", Optical Engineering, Vol. 13, Issue 4, July/August, 1974. In this type of system, once the detector is irradiated, the detector output signal is amplified and processed to form a TV image of the scene or object. The principal shortcoming of the serial scan thermal imager, which has been referred to in the art as a "FLIR" (forward-looking-infrared), is the limited sensitivity afforded by a single detector element. Enhanced sensitivity in serial scanning is acheived by adding elements in a row, then time delaying and integrating the outputs of the elements to acheive higher signal-to-noise ratio at the detector output. Even further enhancement is acheived by adding rows of detectors. The detectors are then scanned in parallel, and the output signal is converted to a serial format for display through processing electronics. The total number of elements in systems of this type range from about 20 to 50.
Another approach to thermal imaging design employs parallel scanning. This type of device has been referred to in the art as a parallel scan FLIR. In this device a linear array of detector elements is scanned over an image such that the vertical dimension of the image spans the length of the array. A "framing" or "scanning" mirror, usually oscillating at about 60 Hz, and an imaging lens direct light rays from the scene onto the linear detector array. The detector array is fixed while the image moves horizontally across the array. At the completion of the scan, the mirror is driven back rapidly to its original position, and the scan cycle is repeated. This process provides a highly efficient unidirectional linear scan which results in two interlaced "fields". A scan converter is used to convert the data into a serial format suitable for television display. Typically, 120 detector elements are used in the linear array, thereby offerring enhanced sensitivity as compared to the serial scan type of device. A typical parallel scan system is also described by Anderson.
In heretofore known thermal imaging systems of the serial and parallel scanning types a "common module" approach has been adopted. In a common module system each of the various elements of the imaging system are constructed as independent, modular components which are assembled and must operate together in order to provide a complete system. The modules typically comprise separate telescope, scanner, imager, detector/dewar, cooling engine, and electronics modules. This approach to thermal imager design is generally described in Preliminary Common Module Designers Handbook, U.S. Night Vision Laboratory, Ft. Belvoir, Va. (Contract: DAAG53-75-C-0718. --Approved for Public Release).
By way of example only, FIG. 1 illustrates the various modular components of a thermal imager constructed in accordance with the common module approach. As there shown, the modular components consist of a telescope, a scanner, an imager, a detector/dewar with an associated cooling source, and electronics modules for amplifying and processing the detector signals. The telescope is of afocal design, and generally consists of a Galilean telescope to provide magnification of the image. In the case of the serial scan FLIR, the scanner unit performs a vertical and horizontal scan in order to create a pixel by pixel scan of the image. In the case of a parallel scan FLIR, the scanner is an oscillating flat mirror which performs a horizontal scan of the image. In either case, the imager consists of three or more lens elements and a folding mirror. The imager receives the output of the scanner and forms an image of the scanned scene on the detector. The detector/dewar consists of a dewar, a sealed flask with its contents under vacuum, and a detector disposed within the dewar. The detector has a flat dewar window, and the imager focuses the image onto the detector through the dewar window. The detector/dewar is coupled to the cryogenic cooling system and receives cold source input so as to cool the detector to cryogenic temperature for optimum performance. In a parallel scan FLIR the detector typically consists of a linear array of about 120 detector elements with output pins for each detector element. The detector converts the image to electrical signals, and preamplifier electronics, typically six circuit boards for a 120 detector array, amplify the electrical outputs of the various elements of the detector array. Signal processing from this point forward in the system depends upon the particular system configuration. A visual display of the infrared scene can be effected via a rather complex arrangement of lenses, light-emitting diodes and timing circuitry. A television display requires scan converter electronics to convert parallel channel outputs into a serial format for presentation in a standard television format.
As the art is developing, great emphasis is being placed on sensitivity enhancement. One way to enhance sensitivity still further is to use a full frame two dimensional array, known as a "staring array". In this type of array no scanning is employed, and each detector element of the array corresponds to a picture element in the scene. In such a system, a lens is used to form the image on the array in a manner similar to an ordinary photographic camera with the array replacing the film at the focal plane. However, manufacturing technology limitations have thus far prevented the fabrication of sufficiently large arrays which can attain adequate performance at acceptable costs.
The demand for still further sensitivity and the limitations of existing manufacturing technology relative to two dimensional arrays have led to the development of newer parallel scan systems employing several times more detector elements. Newer arrays may consist of as many as 1000 detector elements per column, with 1 to 16 columns per array. For such multi-column arrays a time delay and integration operation is performed across the various columns for corresponding detector elements in the columns. Such arrays require detector signal preamplification, multiplexing, and processing directly on the focal plane in order to avoid the need to rout a prohibitively large number of electrical leads out of the detector/dewar. Of course, the detector elements still must be cooled, typically to 77.degree. Kelvin. The basic common module approach is to be retained, to the extent possible, in these more sensitive next generation systems. More particularly, separate scanner, imager, and detector/dewar modules are required. All of these components add to the overall size, weight and complexity of the system.
Thus, there exists a need for an improved thermal imaging system having enhanced system sensitivity while also reducing the bulk, weight and cost of the system.
3. Objects of the Invention
It is therefore one object of the present invention to provide a smaller, more compact, and lighter weight imaging system.
Another object of the invention is to provide a thermal imaging system having improved system sensitivity.
It is a further object of the present invention to provide a simplified thermal imaging system than can presently be attained following the common module approach.
Another object of the invention is to provide a thermal imaging system having improved imaging optics.
Another object of the invention is to provide an optical imaging system having fewer components, thereby reducing the complexity of the system.
These and other objects and advantages are accomplished in a highly compact and light weight integrated imaging system having greatly improved radiation transmission and, hence, system sensitivity. The highly compact imaging system of the present invention provides excellent imaging while also presenting a potential opportunity to reduce imaging system cost.